Biaxially oriented film having a layer consisting of ethylene-vinyl-alcohol copolymer (EVOH)

ABSTRACT

The invention relates to a method for manufacturing a multilayer biaxially oriented film which comprises at least five layers having the arrangement A/B/C/B/A, the inner layer C being synthesized from an ethylene-vinyl-alcohol copolymer (EVOH layer) and an adhesion promoting layer B made of modified polyolefin being applied on both sides on each surface of the EVOH layer C and a layer A made of a partially crystalline thermoplastic polyolefin being applied to the surfaces of the particular adhesion promoting layers, characterized in that the melts corresponding to the individual layers of the film are coextruded through a sheet die, the multilayer film thus obtained is drawn off on one or more roll(s) for solidification, the film is subsequently stretched in the longitudinal direction and then in the transverse direction using a tenter frame, the EVOH layer C and the adhesion promoting layers B and the layer A are coextruded at the same width, and the tenter hooks of the frame engage all five layers jointly and simultaneously during the transverse stretching. The invention also relates to a film, and a packaging made from the film.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. 371)of PCT/EP2003/013438 filed Nov. 28, 2003 which claims benefit to Germanapplication 102 56 110.9 filed Nov. 29, 2002.

The present invention relates to a multilayer film having at least oneinternal layer made of an ethylene-vinyl-alcohol copolymer (EVOH).

Biaxially oriented polypropylene films are known in the related art.These films are used in greatly varying fields, such as food packaging,wrapping cigarettes, laminating film, and technical applications. Thepolypropylene film obtains many important usage properties throughorientation in and perpendicular to the machine running direction, i.e.,biaxial orientation. These boPP films are distinguished by, among otherthings, good transparency, high gloss, and a barrier to water vapor.

The barrier properties of the biaxially oriented polypropylene films tooxygen are in need of improvement, however. Therefore, for manyapplications, the films are coated in an additional processing stepafter manufacturing. Acrylate coatings, PVDC, PVOH, and other materialsare used for this purpose. A further possibility is the metal coating ofboPP films.

Combining the polypropylene layer with a further barrier layer is alsofrequently suggested. The selection of materials which may be combinedwith polypropylene is restricted, however. Therefore, until now only afew film composites have been successfully developed having barrierlayers made of polymers which are different from polypropylene. Forexample, polyester layers, polyamide layers, or layers made ofethylene-vinyl-alcohol copolymer are used. These materials havecompletely different rheological properties than the typicalpolypropylenes, however, and therefore may not be stretched jointlywithout special measures. The adhesion of these layers to one another isalso problematic. Solutions for these problems have already beensuggested in the related art.

Thus, for example, a multilayer stretched polypropylene film having alayer made of ethylene-vinyl-alcohol (EVOH) copolymer is known. U.S.Pat. No. 4,561,920 describes a laminate made of an EVOH copolymer layerand a polymer adhesive layer on at least one surface of the EVOH layerand a polypropylene layer. The polymer adhesive layer is synthesizedfrom polyolefin modified using maleic acid anhydride. According to thisteaching, it is essential to the invention that the EVOH copolymer havea melt-flow index of at least 8 g/10 minutes (190° C. and 2.16 g), sothat biaxial stretching of the composite is possible. Furthermore,according to the description, to achieve a good oxygen barrier, thebiaxially stretched composite must be subjected to thermal fixing. Forthis purpose, the stretched film is finally guided over a row of heatedrollers in order to encourage recrystallization of the EVOH andtherefore improve the barrier values. It is specified that the oxygenbarrier at 20° C. and 0% ambient humidity is approximately 12cm³/m²*day.

EP 0758675 B1 describes a laminate made of an EVOH layer and apolypropylene film. The two layers are bonded to one another using anadhesive layer. The laminate may be produced using coextrusion, in whichthe layers made of EVOH, polypropylene, and adhesion promoter arecoextruded jointly and simultaneously through a tubular die.

The oxygen barrier of these known films is also in need of improvement.Good oxygen barriers are particularly to be provided even in the eventof elevated ambient humidity. Furthermore, it would be desirable to finda method which is more flexible in regard to the usable raw materials.

The object of the present invention is therefore to provide a film whichmay be manufactured on the typical stentering facilities via the processof sequential biaxial orientation and which has a favorable propertyprofile. In this case, the good usage properties, such as gloss,transparency, etc., of the known biaxially oriented polypropylene filmsare to be maintained above all. In addition, good barrier properties inrelation to oxygen and flavoring agents are especially desirable.

This object is achieved by a method for manufacturing a multilayerbiaxially oriented film which comprises at least five layers having thearrangement A/B/C/B/A, the inner layer C being synthesized from anethylene-vinyl-alcohol copolymer (EVOH) and an adhesion promoting layerB made of modified polyolefin being applied on both sides on eachsurface of the ethylene-vinyl-alcohol (EVOH) layer and a layer A made ofa partially crystalline thermoplastic polymer being applied to thesurfaces of the particular adhesion promoting layers, the meltscorresponding to the individual layers of the film being coextrudedthrough a sheet die, the multilayer film thus obtained being drawn offon one or more rollers for solidification, the film subsequently beingstretched in the longitudinal direction and then in the transversedirection using a tenter frame, the ethylene-vinyl-alcohol copolymerlayer C and the adhesion promoting layers B and the layers A beingcoextruded with the same width and the tenter hooks engaging all fivelayers jointly and simultaneously during the transverse stretching. Thedependent subclaims specify preferred embodiments of the presentinvention.

In the scope of the present invention, it has been found that via themethod according to the present invention, in which the tenter hooksengage the layers A, B, and C jointly, the transverse stretching forcesare introduced into the film in such a way that a significantly betterstretching of the EVOH layer together with the remaining layers of thecoextruded composite is possible. It is suspected that the methodresults in a higher stretch-induced crystallization in the layers C andA, since all layers absorb the transverse stretching forces directly andthe stretching forces are not only absorbed by the base layer andtransmitted to the remaining layers, as in the free edge extrusionaccording to the related art. As a result, a better oxygen barrier isachieved than in comparable known constructions, without an additionalthermal recrystallization being necessary. Simultaneously, layers whoserheological properties differ strongly may be stretched with one anothervia the method according to the present invention. Therefore, forexample, it is even possible to use EVOH polymers having a MFI of lessthan 8 g/10 minutes for the internal EVOH layer without the knownfisheye effect occurring. The method according to the present inventiontherefore has two decisive advantages. The films manufactured accordingto this method have a higher oxygen barrier and the materials for theindividual layers may be sought out more flexibly, i.e., also takingother aspects, and not only stretchability, into consideration.

In the following, the composition and synthesis of the individual layerswill be described.

Inner Layer C:

The inner layer C made of EVOH copolymer (referred to the following asthe EVOH layer) contains at least 50 weight-percent, preferably 70 to100 weight-percent, particularly 80 to <100 weight-percent, each inrelation to the layer, of an ethylene-vinyl-alcohol (EVOH) copolymerdescribed in the following. The layer C is referred to as the innerlayer, since further layers are applied to both surfaces of the layer C.

EVOH copolymers are known per se in the related art and are manufacturedthrough saponification or hydrolysis of ethylene-vinyl acetatecopolymers. EVOH copolymers which have a degree of hydrolysis (degree ofsaponification) of 96 to 99% are particularly suitable. Furthermore, theethylene content is to be in the range from 25-75 mole-percent,preferably in the range from 30-60 mole-percent, particularly in therange from 35-50 mole-percent. The melting point is generally in a rangefrom 150 to 190° C. The melt-flow index at 190° and 2.16 g may be in therange from less than 8 g/10 minutes, preferably in the range from 1 to 7g/10 minutes, particularly 2 to 6 g/10 minutes. Surprisingly, it ispossible according to the method according to the present invention tostretch the film composite and achieve very good barrier values even ifsuch an EVOH copolymer is used. In a further embodiment, the melt-flowindex of the EVOH copolymer may even be higher, for example, ≧8 g/10minutes, preferably 10 to 20 g/10 minutes.

The thickness of the EVOH layer is generally 1 to 10 μm, preferably 2 to8 μm, particularly 3 to 6 μm. It has been found that the adhesivestrengths of the layers on one another are also critically dependent onthe thickness of the EVOH layer. It is especially advantageous not toexceed this thickness of 10 μm. With layers which are too thick,delamination of the coextruded layers occurs even after the extrusion onthe cooling roller—depending on the EVOH selected. Stretching of thislayered construction is then no longer possible. Surprisingly, it ispossible according to the method according to the present invention tostretch the film sequentially using comparatively high stretchingfactors, even if the layer thickness of the EVOH layer is over 2 μm.

Adhesion Promoting Layer B:

It is essential according to the present invention that the layer A andthe EVOH layer C are bonded to one another via an adhesion promotinglayer B. The adhesive layer is therefore applied between the innerethylene-vinyl-alcohol (EVOH) layer and the layer made of partiallycrystalline polyolefin A (strength layer), i.e., it is applied on eachsurface of the ethylene-vinyl-alcohol (EVOH) layer. The adhesive layer Bensures that the ethylene-vinyl-alcohol (EVOH) layer C and the layer Aare bonded so solidly to one another that both layers C and A arestretched jointly when they are engaged simultaneously and jointly bythe tenter hooks in the tenter frame, and the ethylene-vinyl-alcohol(EVOH) layer is oriented in such a way that the adhesion of theindividual layers to one another is maintained. The adhesive layer issynthesized from modified polyolefins. In general, the adhesive layercontains at least 90 weight-percent, preferably 95 to 100weight-percent, particularly 99 to <100 weight-percent of the modifiedpolyolefin, in relation to the weight of the adhesive layer in eachcase.

The modified polyolefins are based on ethylene polymers or propylenepolymers, of which propylene homopolymers, propylene copolymers, andpropylene terpolymers are preferred. Propylene copolymers or terpolymerspredominantly contain propylene units, preferably at least 80-98weight-percent, and ethylene and/or butylene units in appropriatequantities as comonomers. These polymers are preferably modified usingmaleic acid anhydride, possibly also using other carboxylic acid unitsor their esters, such as acrylic acid or its derivatives.

Modified polypropylenes and polyolefins of this type are known per se inthe related art and are sold, for example, by Mitsui Chemicals under thetrade name Admer® or by Mitsubishi Chemicals under Modic® or by Chemplexunder Plexar®, and as Epolene® by Eastman. The modified polypropylenesare manufactured from the unmodified polypropylenes and maleic acidanhydride by reacting maleic acid anhydride with polypropylenes ofsuitable viscosity at elevated temperatures. A method is described, forexample, in U.S. Pat. No. 3,480,580. The modification is also referredto as a grafting reaction and the modified polypropylenes arecorrespondingly referred to as graft polymers, which are grafted withmaleic acid anhydride.

For the purposes of the present invention, propylene homopolymers orpropylene copolymers which are modified using maleic acid anhydride(e.g., Q series of Mitsui Chemicals), whose melt-flow index is in therange from 1 to 10 g/10 minutes at 230° C. (ASTM D 1238) and whose Vicatsoftening point is between 110 and 155° C., are preferred.

The thickness of the adhesive layer B is generally 0.4 to 4 μm,preferably 0.5 to 3 μm, particularly 0.8 to 2 μm.

Layer A:

For orientation of a film made of thermoplastic polymer, it is basicallynecessary for stretching forces introduced into the film via rollers ora stretching frame or other suitable means to act on all layers of thefilm in order to result in an orientation of each layer. It has beenfound in the scope of the present invention that a film constructionwhich comprises both a polypropylene layer and an EVOH layer may bestretched in the longitudinal and transverse directions especiallyadvantageously according to the method according to the presentinvention. As already explained, it is essential that in the transversestretching all layers A, B, and C are engaged by the tenter hooks,through which direct absorption of the stretching forces by all layers Aand C is provided.

According to the typical free edge method, the central base layer isextruded broader than the other layers, so that the tenter hooks do notalso engage on the additional layers. Accordingly, in application on thefilm structure provided here, the tenter hooks would only engage thecentral layer made of EVOH, which must absorb the stretching forcesalone and transmit them to the polypropylene layers. It has been shownthat a film may also be manufactured according to this method ifoutstanding adhesion of the individual layers to one another is providedand if comparatively moderate stretching factors are applied. Theselection of the main components for the individual layers is thus veryrestricted, for example, to the selection of an EVOH having an MFI of atleast 8 g/10 minutes or an EVOH having an ethylene content of at least40 mole-percent.

It has been shown from the above explanations that the layer C may alsobe selected from a significantly larger variety than in the methodsknown until now. The layer C must have a sufficiently large adhesivestrength in relation to the adhesive layer B and be suitable forabsorbing stretching forces, i.e., a partially crystalline polyolefin.Otherwise, it must only be ensured that the softening point is not toolow in relationship to the transverse stretching temperature in theframe, so that adhesion of the tenter hooks to this layer C in thetenter frame is prevented. All materials which fulfill the requirementscome into consideration in principle as the polyolefin for the layer C.

Partially crystalline polyolefins, whose crystallinity is at least 10 to70%, preferably 30 to 70%, and whose melting point is at least 140° C.are suitable for the layer C. Preferably, a polymer whose ethylenecontent is between 0 and 5 weight-percent in relation to the polymer isused. Isotactic propylene homopolymers having a melting point from 150to 170° C., preferably from 155 to 165° C., and a melt-flow index(measurement DIN 53 735 at 21.6 N load and 230° C.) from 1.0 to 15 g/10minutes, preferably from 1.5 to 8 g/10 minutes, are especially suitable.The n-heptane soluble component of the isotactic propylene homopolymersis generally 1 to 10 weight-percent, preferably 2 to 5 weight-percent,in relation to the starting polymer. The crystallinity of the propylenehomopolymers is preferably 40 to 70%, particularly 50 to 70%. Themolecular weight distribution of the homopolymers may vary. The ratio ofthe weight average M_(w) to the number average M_(n) is generallybetween 1 and 15, preferably 2 to 10, very especially preferably 2 to 6.A narrow molecular weight distribution of the propylene homopolymers ofthis type is achieved, for example, by its peroxidic degradation or bymanufacturing the polypropylene using suitable metallocene catalysts.

The layer thickness of the layer C is preferably 5 to 15 μm, morepreferably 6 to 10 μm. It has been found that at layer thicknesses ofbelow 5 μm, the stretching becomes more difficult and the composite maystill only be biaxially oriented poorly. At layer thicknesses of above15 μm, the overall thickness of the film is unfavorable, although formany applications the overall thickness of the film is not subject toany upper limit.

In a further embodiment, the layer C may be an opaque layer, as isprovided in known opaque boPP films as the opaque base layer. In thisembodiment, the layer C is opaque due to the addition of fillers. Ingeneral, the layer C in this embodiment contains at least 70weight-percent, preferably 75 to 99 weight-percent, particularly 80 to98 weight-percent, each in relation to the weight of the layer C, of oneof the partially crystalline polyolefins and/or propylene polymersdescribed above for the layer C, the propylene homopolymers describedalso being preferred.

The opaque C contains additional fillers in a quantity of at most 30weight-percent, preferably 1 to 25 weight-percent, particularly 2 to 20weight-percent, in relation to the weight of the layer C. According tothe present invention, fillers are pigments and/or vacuole-initiatingparticles and are known per se in the related art.

Pigments are incompatible particles which essentially do not result invacuole formation upon stretching of the film. The pigmenting effect ofthe pigments is caused by the particles themselves. “Pigments” generallyhave an average particle diameter of 0.01 to at most 1 μm, and compriseboth “white pigments”, which color the films white, and also “colorpigments”, which provide the film with a colored or black color. Typicalpigments are materials such as aluminum oxide, aluminum sulfate, bariumsulfate, calcium carbonate, magnesium carbonate, silicates such asaluminum silicate (kaolin clay) and magnesium silicate (talc), silicondioxide, and titanium dioxide, of which white pigments such as calciumcarbonate, silicon dioxide, titanium dioxide, and barium sulfate arepreferably used.

“Vacuole-initiating fillers” are solid particles which are incompatiblewith the polymer matrix and lead to the formation of vacuole-likecavities when the film is stretched, the size, type, and number of thevacuoles being a function of the quantity and size of the solidparticles and the stretching conditions such as the stretching ratio andstretching temperature. The vacuoles reduce the density and provide thefilm with a characteristic nacreous, opaque appearance, which arises dueto light scattering at the boundaries “vacuole/polymer matrix”.Typically, the vacuole-initiating fillers have a minimum size of 1 μm,in order to lead to an effective, i.e., opaque-making quantity ofvacuoles. In general, the average particle diameter of the particles is1 to 6 μm, preferably 1.5 to 5 μm.

Typical vacuole-initiating fillers are inorganic and/or organicmaterials which are incompatible with polypropylene, such as aluminumoxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesiumcarbonate, silicates such as aluminum silicate (kaolin clay) andmagnesium silicate (talcum) and silicon dioxide, of which calciumcarbonate and silicon dioxide are preferably used. The typically usedpolymers which are incompatible with the polymers of the base layer comeinto consideration as organic fillers, particularly HDPE, copolymers ofcyclic olefins such as norbornene or tetracyclododecene with ethylene orpropene, polyesters, polystyrenes, polyamides, and halogenated organicpolymers, with polyesters such as polybutylene terephthalate beingpreferred. “Incompatible materials and/or incompatible polymers” means,according to the present invention, that the material and/or the polymerexist in the film as separate particles and/or as a separate phase.

The opaque layer C generally contains pigments in a quantity from 0.5 to10 weight-percent, preferably 1 to 8 weight-percent, particularly 1 to 5weight-percent. Vacuole-initiating fillers are generally contained in aquantity from 0.5 to 30 weight-percent, preferably 1 to 15weight-percent, particularly 1 to 10 weight-percent. The specificationsrelate to the weight of the layer C.

The density of the opaque layer C and therefore the film may varydepending on the composition of the opaque layer C in a range from 0.4to 1.1 g/cm³. In this case, vacuoles contribute to reducing the density,while in contrast pigments such as TiO₂ elevate the density of theopaque layer because of their higher specific weight. The density of theopaque layer in opaque embodiments is preferably 0.5 to 0.95 g/cm³.

It has been found that even opaque layers, particularly even thosehaving a vacuole-containing structure, are suitable as a layer fortransmitting the stretching forces and therefore for manufacturing astretched composite. The layer thickness of the opaque layer C ispreferably in the range from 5 through 15 μm even for these embodiments.

In a preferred embodiment, the film according to the present inventionhas, in addition to the construction made of EVOH layer C, adhesivelayers B, and layers C, at least one surface layer, preferably layers onboth sides, which is/are applied to the surface(s) of the layers C.Six-layered and seven-layered film structures are thus implemented.These polyolefinic surface layers are then the external layers of themultilayer film construction and determine specific functionalities,such as sealability, gloss, friction, and other properties of the film,which are a function of the external layers. The surface layers aregenerally synthesized from polymers made of olefins having 2 to 10carbon atoms. The surface layers generally contain 95 to 100weight-percent polyolefin, preferably 98 to <100 weight-percentpolyolefin, in relation to weight of the particular surface layer.

Examples of suitable olefinic polymers of the surface layers arepolyethylenes, polypropylenes, polybutylenes, or mixed polymers made ofolefins having two to eight C atoms, of which copolymers or terpolymersmade of ethylene, propylene, and/or butylene units or mixtures of thepolymers cited are preferred. These olefinic polymers preferably containno functional groups and are synthesized solely from olefinic monomers.Preferred mixed polymers are

-   -   random ethylene-propylene copolymers, preferably having an        ethylene content of 1 to 10 weight-percent, particularly 2.5 to        8 weight-percent, or    -   random propylene-butylene-1 copolymers, preferably having a        butylene content of 2 to 25 weight-percent, more preferably 4 to        20 weight-percent, or    -   random ethylene-propylene-butylene-1 terpolymers, preferably        having an ethylene content of 1 to 10 weight-percent and a        butylene-1 content of 2 to 20 weight-percent, or    -   a mixture or a blend of ethylene-propylene-butylene-1        terpolymers and propylene-butylene-1 copolymers having an        ethylene content of 0.1 to 7 weight-percent and a propylene        content of 50 to 90 weight-percent and a butylene-1 content of        10 to 40 weight-percent.

The specifications in weight-percent each relate to the weight of thecopolymers or terpolymers. The copolymers and/or terpolymers used in thesurface layers described above, which are only synthesized from olefins,generally have a melt-flow index of 1.5 to 30 g/10 minutes, preferablyof 3 to 15 g/10 minutes. The melting point is in the range from 120 to140° C. The blend described above made of copolymers and terpolymers hasa melt-flow index of 5 to 9 g/10 minutes and a melting point from 120 to150° C. All melt-flow indices specified above were measured at 230° C.and a force of 21.6 N (DIN 53 735).

Suitable polyethylenes for the surface layers are HDPE, MDPE, and LDPE,as are typically used in biaxially oriented packaging films.

The thickness of the particular surface layer is generally greater than0.1 μm and is preferably in the range from 0.5 to 10 μm, particularly 1to 5 μm.

The surface layers and/or the layer C may contain additional typicaladditives such as neutralization agents, stabilizers, antistatic agents,antiblocking agents, and/or lubricants in effective quantities in eachcase. The following specifications in weight-percent each relate to theweight of the particular surface layer.

Suitable antiblocking agents are inorganic additives such as silicondioxide, calcium carbonate, magnesium silicate, aluminum silicate,calcium phosphate, and the like, and/or incompatible organic polymerssuch as polyamides, polyesters, polycarbonates, and the like, orcross-linked polymers such as cross-linked polymethyl methacrylate orcross-linked silicone oils. Silicon dioxide and calcium carbonate arepreferred. The average particle size is between 1 and 6 μm, particularly2 and 5 μm. The effective quantity of antiblocking agent is in the rangefrom 0.1 to 5 weight-percent, preferably 0.5 to 3 weight-percent,particularly 0.8 to 2 weight-percent.

Preferred antistatic agents are alkali-alkane sulfonates,polyether-modified, i.e., ethoxylated and/or propoxylated polydiorganicsiloxanes (polydialkyl siloxanes, polyalkyl phenyl siloxanes, and thelike) and/or the essentially straight-chain and saturated aliphatic,tertiary amines having an aliphatic residue having 10 to 20 carbonatoms, which are substituted with ω-hydroxy-(C₁-C₄)-alkyl groups, N,N-bis-(2-hydroxyethyl)-alkyl amines having 10 to 20 carbon atoms,preferably 12 to 18 carbon atoms in the alkyl residue being especiallysuitable. The effective quantity of antistatic agent is in the rangefrom 0.05 to 0.5 weight-percent.

Lubricants are higher aliphatic acid amides, higher aliphatic acidesters, waxes, and metal soaps, as well as polydimethyl siloxanes. Theeffective quantity of lubricant is in the range from 0.01 to 3weight-percent, preferably 0.02 to 1 weight-percent. The addition ofhigher aliphatic acid amides in the range from 0.01 to 0.25weight-percent to the base layer is especially suitable. Particularlysuitable aliphatic acid amides are erucic acid amide and stearyl amide.The addition of polydimethyl siloxanes in the range from 0.02 to 2.0weight-percent is preferred, particularly polydimethyl siloxanes havinga viscosity from 5,000 to 1,000,000 mm²/second.

The typical stabilizing compounds for ethylene, propylene, and otherα-olefin polymers may be used as stabilizers. The quantity added isbetween 0.05 and 2 weight-percent. Phenolic and phosphitic stabilizersare especially suitable. Phenolic stabilizers having a molecular mass ofmore than 500 g/mol are preferred, particularlypentaerythrityl-tetrakis-3-(3,5-di-tertiarybutyl-4-hydroxyphenyl)-propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tertiarybutyl-4-hydroxybenzyl)benzene. In this case, phenolic stabilizers areused alone in a quantity from 0.1 to 0.6 weight-percent, particularly0.1 to 0.3 weight-percent, phenolic and phosphitic stabilizers are usedin the ratio 1:4 to 2:1 and in a total quantity of 0.1 to 0.4weight-percent, particularly 0.1 to 0.25 weight-percent.

Neutralization agents are preferably dihydrotalcite, calcium stearate,and/or calcium carbonate of an average particle size of at most 0.7 μm,an absolute particle size of less than 10 μm, and a specific surfacearea of at least 40 m²/g.

The total thickness of the film according to the present invention mayvary within wide limits and depends on the intended use. It is typically4 to 100 μm, particularly 5 to 80 μm, preferably 6 to 60 μm.

In one possible embodiment, the surface(s) of the layer(s) C or of theadditional surface layer(s) is/are corona, plasma, or flame treated.This treatment elevates the adhesion to printing inks, adhesives,cold-sealing layers, metal layers, etc., in a way known per se.

In the following, the individual method steps of the method according tothe present invention will be described in greater detail:

In the scope of this method, in principle, the melts corresponding tothe individual layers of the film are coextruded through a sheet die,the film thus obtained is drawn off on one or more rollers forsolidification, the film is subsequently stretched (oriented), and thestretched film is fixed and possibly plasma, corona, or flame treated onthe surface layer provided for treatment. The biaxial stretching(orientation) is performed sequentially, the stretching first beingperformed longitudinally (in the machine direction) and thentransversely (perpendicular to the machine direction).

As is typical in the extrusion method, the polymer and/or the polymermixture of the individual layers is compressed in an extruder andliquefied, the additives possibly added already able to be contained inthe polymer and/or in the polymer mixture. The melts of the differentlayers are laid one on top of another and combined as melt flowsaccording to the related art and then pressed jointly and simultaneouslythrough a sheet die. The individual layers may be combined in differentregions of the die, so that from a chronological viewpoint, the meltsare laid one on top of another at different instants.

According to the related art, multilayer films are typically extrudedaccording to the free edge principle, i.e., the width of the surfacelayers is reduced in relation to the width of the base layer, due towhich the two edge regions of the film web remain free of surface layermaterial. According to the free edge method, the support tenter hooks ofthe tenter frame engage the film at this free edge and thus transmit thestretching forces directly to the central base layer of a film. Themethod according to the present invention is very advantageous inrelation to this related art.

It is essential for the method according to the present invention thatthe layer C, in contrast to the typical free edge method described, isextruded having the same width, and/or approximately the same width, asthe adhesive layers B and the EVOH layer A lying underneath. This isessential so that the tenter hooks may engage the layers C and theadhesion promoter layers B and the EVOH layer simultaneously during thetransverse stretching. It has been shown that the simultaneous action ofthe tenter hook forces on the composite A/B/C/B/A allows a significantlymore uniform and higher stretching of all layers and the EVOH layer maythus be stretched together with the layer C.

In addition, it has been found that maintaining specific extrusionconditions is especially advantageous. It has been found that theadhesion of the individual layers to one another, particularly theadhesion of the EVOH layer to the adhesive layer C, is a function of thedwell time of the melts laid one on top of another. It contributes togood adhesion if the individual layers of the film, which have alreadybeen laid one on top of another, dwell for a specific period of time inthe molten state laid one on top of another before exiting the die, sothat an intensive bonding between the individual layers is achieved.

Therefore, in a further embodiment, it is especially advantageous forthe method according to the present invention to ensure a dwell time ofthe molten layers, particularly a dwell time of the melts of theadhesion promoting layers on the EVOH melt, of at least 6 seconds in thedie. The longer this dwell time of the layers laid one on top ofanother, particularly the adhesion promoting melts on the EVOH melt, isextended in the die, the better the adhesion of the layers. This dwelltime is preferably 8 to 180 seconds, particularly 8 to 100 seconds. Ithas been observed that in the event of too short dwell times of themolten layers, particularly the adhesion promoting layers on the EVOHlayer, in the die, delaminations may occur more easily during thesubsequent biaxial orientation of the coextruded composite, inparticular, sufficient adhesive strength is no longer provided duringtransverse stretching under certain circumstances. The lack of adhesionresults in the ethylene-vinyl-alcohol (EVOH) layer not being oriented,due to which cracks arise in the layer, which may be perceivedmacroscopically as massive visual defects. The films also have nobarrier to oxygen.

The dwell time of the melts in the die may basically be controlled viathe die geometry and the extruder output. A die extended in the mainflow direction (transversely to the die lip) extends the dwell time. Alower extruder output also extends the dwell time in connection with theappropriately adapted draw-off and running speeds of the draw-offrollers.

Of course, it is necessary to heat the die as usual, so that the meltslaid one on top of another are kept at the required temperature. The dietemperature is typically 200 to 300° C., preferably 210-250° C.

The multilayer melt guided in this way is shaped into a flat film in thedie and drawn off on one or more draw-off rollers at a temperature of 10to 100° C., preferably 10 to 60° C., immediately after exiting the die,so that it cools to a multilayer precursor film and solidifies.

The precursor film thus obtained is then stretched longitudinally andtransversely to the extrusion direction. The longitudinal stretching ispreferably performed at a temperature of 110 to 165° C., preferably 120to 160° C., particularly 140 to 160° C., expediently with the aid of tworolls running at different speeds in accordance with the stretchingratio desired. The longitudinal stretching ratios are in the range from2 to 8, preferably 3 to 6, particularly 4 to 6. Surprisingly, stretchingfactors of more than 4.5, which are typical when stretching boPP films,may be applied according to the method according to the presentinvention.

The transverse stretching is preferably performed at a temperature of130 to 180° C., preferably 140 to 180° C., with the aid of acorresponding tenter frame. As already noted, it is essential in thiscase that the composite made of the layers A/B/C/B/A is engaged jointlyand simultaneously by the tenter hooks of the tenter frame. Thetransverse stretching ratios are in the range from 3 to 10, preferably 5to 9.

According to the present invention, high area stretching ratios(longitudinal stretching factor*transverse stretching factor) of greaterthan 20, preferably 24 to 50, particularly 25 to 40, may be implementedfor these composites according to the sequential method.

Typical fixing to reduce shrinkage tendencies follows the stretching ofthe film if necessary. For this purpose, the film is guided convergingthrough the frame outlet at a controlled temperature. This has nothingto do with the targeted thermal post-treatment for recrystallization, inwhich the film is first cooled and then heated to an elevatedtemperature via heated rollers after the transverse stretching. Finally,the film is wound up in a typical way using a winding device.

Thermal post-treatment for recrystallization of the EVOH layer toimprove the barrier is not necessary according to the present invention,but may nonetheless be expedient for other reasons. In general, such athermal post-treatment at elevated temperature is dispensed with.

Preferably, after the biaxial stretching, one or both surfaces of thefilm is/are plasma, corona, or flame treated according to one of theknown methods. The treatment intensity is generally in the range from 35to 50 mN/m, preferably 37 to 45 mN/m.

For the corona treatment, the film is guided between two conductorelements used as electrodes, such a high voltage being applied betweenthe electrodes, usually alternating voltage (approximately 5 to 20 kVand 5 to 30 kHz), that spray or corona discharges may occur. Through thespray or corona discharge, the air above the film surface is ionized andreacts with the molecules of the film surface, so that polarintercalations arise in the essentially nonpolar polymer matrix.

If no other specifications were made in the description, the followingmeasurement methods were used to characterize the raw materials and thefilms:

Melt-Flow Index

The melt-flow index was measured according to DIN 53735 at 21.6 N loadand 230° C.

Melting Point

DSC measurement, maximum of the melting curve, heating speed 20°C./minute.

Turbidity

The turbidity of the film was measured in accordance with ASTM-D1003-52.

Gloss

The gloss was determined according to DIN 67530. The reflector value wasmeasured as the optical characteristic for the surface of a film. On thebasis of the norms ASTM-D 523-78 and ISO 2813, the angle of incidence isset at 60° or 85°. A light beam is incident at the set angle ofincidence on the flat testing surface and is reflected and/or scatteredtherefrom. The light beams incident on the photoelectronic receiver aredisplayed as the proportional electrical variable. The measured value isdimensionless and must be specified with the angle of incidence.

Surface Tension

The surface tension was determined via the ink method (DIN 53364).

Water Vapor and Oxygen Permeability

The water vapor permeability was determined in accordance with DIN 53122part 2. The oxygen barrier effect was determined in accordance with DIN53380 part 3 at an ambient humidity of approximately 50%.

The present invention will now be described in greater detail on thebasis of exemplary embodiments:

EXAMPLE 1

At a die temperature of 240° C., a five-layered film consisting of abase layer C made of ethylene-vinyl-alcohol (EVOH) having adhesionpromoting layers B on both sides and polyolefin layers A applied on bothsides was coextruded together. In this case, all layers were extrudedhaving the same width (no free edge). Subsequently, the melts were drawnoff on a draw-off roller and oriented step-by-step in the longitudinaland transverse directions. The thickness of the layers A wasapproximately 8 μm each, the thickness of the adhesive layers B wasapproximately 0.8 μm each, and the thickness of theethylene-vinyl-alcohol (EVOH) layer C was 5 μm, corresponding to a totalfilm thickness of approximately 23 μm.

Base Layer C:

-   100 weight-percent EVOH (EVAL ES104B) having 44 mole-percent    ethylene content and having a melting point Tm of 156° C. and a    melt-flow index of 6.5 g/10 minutes [at 230° C.; 21.6 N]    Adhesion Promoting Layers B:-   100 weight-percent polypropylene modified using maleic acid    anhydride having a melting point Tm of 160° C. and a melt-flow index    of 7 g/10 minutes to 8 g/10 minutes [at 230° C., 21.6 N] (type ADMER    QF)    Layers A:-   100 weight-percent isotactic propylene homopolymer having a melting    point of 162° C. and a crystallinity of 60% and a melt-flow index of    6.0 g/10 minutes

The manufacturing conditions in the individual method steps were:

Extrusion temperatures: base layer C: 220° C. adhesion promoting layerB: 190° C. layers A: 240° C. temperature of the draw-off roller:  30° C.longitudinal stretching: temperature: 155° C. longitudinal stretchingratio: 4.0-5.0 transverse stretching: temperature: 170° C. transversestretching ratio:  6.8 fixing: temperature: 168° C. convergence: 10%

The transverse stretching ratio λ_(Q)=6.8 is an effective value. Thiseffective value is calculated from the final film width B, reduced bytwo times the cutoff edge width b, divided by the width of thelongitudinal a stretch film C, also reduced by two times the cutoff edgewidth b. The oxygen barrier was 17 cm³/m²*day*bar. The water vaporbarrier was 10.7 g/m²*d.

EXAMPLE 2

A film was manufactured as described in Example 1. In contrast toExample 1, 100 weight-percent Soranol AT 4403 was used as the EVOHpolymer in the layer C. The EVOH had an ethylene content of 44mole-percent and a melt-flow index of 3-4 g/10 minutes (210° C. and 2.16kg) and a melting point of 164° C. A polypropylene Tymor 220 fromMorton, modified using maleic acid anhydride, having a melt-flow indexof 6 g/10 minutes (230° C., 16 kg) and a melting point of 163° C., wasused as the adhesion promoter. The method conditions, as well as thelayer thicknesses and extrusion widths of the individual layers, werenot altered except for the transverse stretching factor. The transversestretching was 8.5 in this example. In this way, a film having an oxygenbarrier of approximately 5 cm³/m²*day*bar was obtained.

EXAMPLE 3

A film was manufactured as described in Example 1. In contrast toExample 1, a different EVOH was used in the central layer. This EVOH hadan ethylene content of 32 mole-percent, a melting point of approximately140° C., and an MFI of 4.5 g/10 minutes. The remaining composition ofthe layers, the layer thicknesses, and the method conditions accordingto Example 1 were not changed. The film thus obtained had an oxygenbarrier of 10.5 cm³/m²*day*bar.

COMPARATIVE EXAMPLE 1

A film was manufactured as described in Example 1. In contrast toExample 1, the EVOH layer was extruded approximately 5% wider than theremaining layers, so that in the transverse stretching only the EVOHlayer was engaged by the tenter hooks. The film showed strong crackingand optical defects (fisheyes). The oxygen barrier was over 300cm³/m²*day*bar. The method was not suitable for stretching the filmcomposite.

1. A method for manufacturing a multilayer biaxially oriented film whichcomprises at least five layers having the arrangement A/B/C/B/A, theinner layer C being synthesized from an ethylene-vinyl-alcohol copolymer(EVOH layer) and an adhesion promoting layer B made of modifiedpolyolefin being applied on both sides on each surface of the EVOH layerC and a layer A made of a partially crystalline thermoplastic polyolefinbeing applied to the surfaces of the particular adhesion promotinglayers, characterized in that the melts corresponding to the individuallayers of the film are coextruded through a sheet die, the multilayerfilm thus obtained is drawn off on one or more roll(s) forsolidification, the film is subsequently stretched in the longitudinaldirection and then in the transverse direction using a tenter frame, theEVOH layer C and the adhesion promoting layers B and the layer A arecoextruded at the same width, and the tenter hooks of the frame engageall five layers jointly and simultaneously during the transversestretching and wherein the EVOH layer has a thickness of at most 10 μm.2. The method according to claim 1, characterized in that the modifiedpolyolefin of the layers B is a polypropylene or polyethylene modifiedusing maleic acid anhydride.
 3. The method according to claim 2,characterized in that the polypropylene modified using maleic acidanhydride has a melt-flow index of 1 to 10 g/10 minutes and a softeningpoint between 110 and 155° C.
 4. The method according to claim 1,characterized in that the adhesion promoting layers each have athickness of 0.4 to 4 μm.
 5. The method according to claim 1,characterized in that the EVOH copolymer has an ethylene content of30-60 mole-percent ethylene and a melting point in the range from140-190° C.
 6. The method according to claim 5, characterized in thatthe EVOH copolymer has a melt-flow index of 1-7 g/10 minutes.
 7. Themethod according to claim 1, characterized in that the polyolefin of thelayers A has a melting point of 150-170° C.
 8. The method according toclaim 7, characterized in that the polyolefin of the layers A is anisotactic propylene homopolymer having a melting point of 155-165° C. 9.The method according to claim 1, characterized in that at least one ofthe two layers A contains vacuole-initiating fillers and/or pigments inaddition to the partially crystalline polyolefin and is opaque.
 10. Themethod according to claim 1, characterized in that the layers A are each5 to 15 μm thick.
 11. The method according to claim 1, characterized inthat a surface layer is applied to at least one surface of the layers Aand this/these surface layer(s) is/are sealable.
 12. The methodaccording to claim 1, characterized in that the melts corresponding tothe adhesion promoting layers and the melt corresponding to the EVOHlayer lie one on top of another in the molten state for a duration of atleast 6 seconds before exiting the die.
 13. The method according toclaim 1, characterized in that the orientation in the longitudinaldirection is performed using a longitudinal stretching ratio of 3:1 to8:1 and the orientation in the transverse direction is performed using atransverse stretching ratio of 3:1 to 10:1.
 14. A method formanufacturing a multilayer biaxially oriented film which comprises atleast five layers having the arrangement A/B/C/B/A, the inner layer Cbeing synthesized from an ethylene-vinyl-alcohol copolymer (EVOH layer)and an adhesion promoting layer B made of modified polyolefin beingapplied on both sides on each surface of the EVOH layer C and a layer Amade of a partially crystalline thermoplastic polyolefin being appliedto the surfaces of the particular adhesion promoting layers,characterized in that the melts corresponding to the individual layersof the film are coextruded through a sheet die, the multilayer film thusobtained is drawn off on one or more roll(s) for solidification, thefilm is subsequently stretched in the longitudinal direction and then inthe transverse direction using a tenter frame, the EVOH layer C and theadhesion promoting layers B and the layer A are coextruded at the samewidth, and the tenter hooks of the frame engage all five layers jointlyand simultaneously during the transverse stretching and wherein the EVOHcopolymer has a melt-flow index of 1-7 g/10 minutes.
 15. A method formanufacturing a multilayer biaxially oriented film which comprises atleast five layers having the arrangement A/B/C/B/A, the inner layer Cbeing synthesized from an ethylene-vinyl-alcohol copolymer (EVOH layer)and an adhesion promoting layer B made of modified polyolefin beingapplied on both sides on each surface of the EVOH layer C and a layer Amade of a partially crystalline thermoplastic polyolefin being appliedto the surfaces of the particular adhesion promoting layers,characterized in that the melts corresponding to the individual layersof the film are coextruded through a sheet die, the multilayer film thusobtained is drawn off on one or more roll(s) for solidification, thefilm is subsequently stretched in the longitudinal direction and then inthe transverse direction using a tenter frame, the EVOH layer C and theadhesion promoting layers B and the layer A are coextruded at the samewidth, and the tenter hooks of the frame engage all five layers jointlyand simultaneously during the transverse stretching and wherein thepolyolefin of the layers A has a melting point of 150-170° C.